On the symbiotic origin of protists, their diversity, and their pivotal role in teaching systematic biology

All multicellular organisms - animals, fungi and plants - are derived from unicellular protists which themselves arose from symbiotic associations of various prokaryotes. The position of protists in relation to other living beings and their tremendous genetic diversity is discussed in relation with studies by Sogin and others on comparative sequencing of small subunit ribosomal RNA. Current views on the symbiotic origin of protists and their organelles (nucleus, 9 + 2 flagellum, mitochondria and plastids) are reviewed with particular reference to extant model organisms from phototrophic dinoflagellates and cryptomonads which are examples of ''eukaryotes within eukaryotes''. It is emphasised that in addition to mutation and selection, symbiosis is an equally important motor of evolution in general. Protists currently seem to fall into three major groups. The early group seems to lack mitochondria primarily and is represented by the Microspora. Giardia, certain amoeboflagellates, trichomonads and polymastigotes. But recent discovery of mitochondrial chaperons throws some doubts on this view. The second, middle group contains the Euglenozoa (kinetoplastids, euglenoids) and the majority of the former rhizopods. The last one, called crown group, due to a rapid radiation, gave rise to the alveolates (which contain the apicomplexean sporozoans and the dinoflagellates, forming together the sister-group of the ciliates), the heterokont algae including brown algae, plus oomycetes and net-slime molds, the cryptomonads, the haptomonads, and the three main branches with multicellular organisms, e.g. plants and animals and true fungi, the latter being the sister group of the animals. Finally, some ideas are developed for a unifying concept of reaching ''protistology'' in modern systematic biology. Protists do nor form a monophyletic taxon, but exemplify a particular, unicellular organisational level; their classification should avoid any hierarchical categories, but search for true sister-group relationships.